The line itself is what casts the tip, leader, and fly. If it would be a D-loop cast, then the line in the loop that is bending the rod is what loads it to throw the rest of the system, the anchor. Others mentioned this above. I understand that a part of the reason for variations in suggested grain weight to given fly rod weight goes back to casting methods, like the continual motion style or the D-loop loaded style. Lighter grain weights that are suggested in some of the micro Skagit line systems are different primarily because of the different casting methods. In a continual motion cast it is the motion and tension of the cast that effects how the rod loads, which requires less weight. Not sure if tip weight plays a role in that. I thought that it is the constant tension without pause as opposed to a pause to form a cast that depends more on the line’s weight without the tip having much of a significant bearing on load.